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Re: [PSUBS-MAILIST] Dome Calculations
Hi Ian.
Thanks for your detailed message and advice. I'll reply to the points below.
>Hi James,
>I'm also planning on using sectional spherical viewport domes.
>While I've not built any, I'll share what I've found:
>1) You could use a neoprene gasket instead of the sealant.
Yes, i think that might be a better idea.
>Also, in the cross-section, where the viewport, the retaining ring
>and the frame meet, there is a little triangle shaped void, you could
>place an O-ring in there to stop water entering the viewport frame
>and retain the dome.
Or perhaps a sika bead? Might be just enough to seal and retain, but still allow removal of the dome.
>Alternatively, the "flat gasket" you've currently drawn (between
>the viewport and retaining ring) could be extended so that it's
>sandwiched between the frame and retaining ring as well as
>sandwiched between the viewport and retaining. This would prevent
>water entering the frame. Figure 11.76 on page 524 of the Stachiw
>book has diagrams of both implementations.
Yes, that is another possibility. I'll check on those pages again.
>2) Yes, you should have an over hang so the inside viewport is no
>where near the edge of the fame. The view port will compress
>slight under operating conditions, fattening the base a little.
>If the base moves over the edge, then there exists the conditions
>for notch cracking to start, which means replacing the viewport.
>In non-normal operating conditions, i.e. the viewport is failing,
>the base will get much bigger, if it goes over the edge, then
>notch cracking will make the dome fail much quicker.
Point taken. I will modify the design to accomodate an overhang.
>I think there is a hint at how much over hang to have in the
>Stachiw book, but I can't find it just now.
I'll see if i can find it.
>3) I've had a hard time finding information about how to design
>the viewport frame/housing. Most are designed thick so that the
>pressure hull moving as it dives deeper will not deform the
>viewport frame/housing/seat. If the frame deforms, then the
>Acrylic will not be against a flat surface and all sorts of other
>problems will come into effect from that.
Thicker is probably better. I'll thicken up the whole thing.
>Acrylic, in some case 2x or 3x.
Its going to be a pretty thick ring if i make the side walls 31mm the same as the acrylic. I'll check it out again in the book.
>As for kicking the dome out in an emergency, my guts say that
>isn't going to work so easily. Alec Smythe had a hydrolic
>piston system for opening his dome in an emergency. Maybe
>make the dome a hatch, so that the frame swings open and things
>like retaining rings, O-rings, sealant and gaskets won't be
>an issue. Will the dome be above the water when it's getting
>kicked off?
Yes, i think i agree with you. Making it a hatch is also a possibility. I will keep thinking.
>I know at least one person who has/had a sub where the dome
>didn't have any retaining device other than water pressure.
How did it stay on at low pressure?
Anyway, thanks very much for your input and i will have another think about the design.
Kind regards
James
Cheers,
Ian.
-----Original Message-----
>From: James Frankland <james@guernseysubmarine.com>
>Sent: Aug 26, 2008 10:02 AM
>To: personal_submersibles@psubs.org
>Subject: [PSUBS-MAILIST] Dome Calculations
>
>Hello Everyone
>
>Progress on my sub has been slow recently, but i have managed to get a few things done including a change to the possible design for the removable front dome mounting system that i am adding instead of the 15" flat port specified on the plans.
>
>I have made some changes to my original design as a result of reading Staichows book and with some advice from members of the group.
>
>http://www.guernseysubmarine.com/index_files/Page12229.htm
>
>I would really appreciate if you could take the time to have a look and let me know what you think of this idea.
>
>Points to note\questions.
>
>1. The "O" ring groove has gone and been replaced with a 1mm bead of sikaflex sealant. Bearing in mind the idea of this design change is so the dome can come out in an emergency, do you think this amount of sealant will stick it permenantly and prevent removal, even with a good kick from the inside? Could make it thinner, 0.5mm?
>
>2. The inside diameter of the ring meets exactly the inside diameter of the dome. (The 32mm bit on the second picture). Should it be a bit larger? 35mm?
>
>3. I do not have the ability to do FEA on this ring, so based on the "rule of thumb" that says you should replace the amount of material you remove, i have calculated volumes of the ring and the removed piece. The figures are shown at the bottom of the page. The endcaps are spun and thicker than specified on the plans. 13mm at the rims ranging to about 10mm at the centre. So, exact calculations for volume of this are difficult. If based on the worst case scenario of 13mm all over, the volume of weld in ring does not cover the "rule of thumb". Should i add an extra 10mm to the thickness of the ring, bearing in mind the endcaps are already quite a bit thicker than needed.
>
>Does that make sense? I've not done one of these before so i think its better to ask silly questions now and avoid mistakes or accidents in the future.
>
>Again, this is still just an idea at this stage and could all change, depending on how things turn out.
>
>I appreciate any advice, good or bad.
>
>Regards
>James
>
>
>
>
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